The so-called insoluble drugs which are insoluble or only sparingly soluble in water and organic solvents are generally difficult to formulate into pharmaceutical preparations. Thus, they are usually made into administrable forms by such techniques as emulsification, clathration, preparation of soluble derivative, entrapping in liposomes, microencapsulation, or the like. However, even by such procedures it is generally still difficult to obtain preparations that would allow the drug to display its action fully.
The present inventors conducted an intensive study to develop a method of solubilizing such drugs and processing them into useful pharmaceutical preparations. The study led to the present invention, i.e., the finding that if a drug-containing phospholipid film, prepared by a process similar to that employed in liposome-producing technology is suspended in an aqueous solution, ultrasonicated and centrifuged, there is obtained a fraction rich in said water-insoluble drug in the lowermost layer of the sediment and that this fraction comprises a drug-phospholipid complex which is water-soluble and very useful as a pharmaceutical product.